Researchers say they've finally solved the decades-old mystery behind a suspicious sound that baffled scientists for years because it's similar to duck quacking and, thanks to sonar technology, has been heard every winter in the waters off Antarctica since the 1960s.

Those who knew of the strange apparent creature calls dubbed the phenomenon "'Bio-Duck," a fitting description because "it really does sound a lot like a duck," Denise Risch, a marine acoustic specialist at the U.S. National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center, said in a story by National Geographic. The noise was so deep and regular that researchers weren't even sure initially it was produced by a living thing.

Some all the way hypothesized the "bio-duck" could actually be the product of an ultra-high-tech devices at the bottom of the ocean, maybe placed there as part of some military operation.

Regardless, as the years flew by, an accumulation of evidence -- like the fact the sound started every year around October and ceased near the end of December and it was detected only in a limited area near Antarctica -- strongly suggested the culprit was a migrating animal.

Now, analysis of calls between Antarctic minke whales (Balaenoptera bonaerensis) reveal the marine mammals were the source of the sound.

Only a truly massive marine animal could produce a sound like that, Risch said. When other scientists asked her to analyze acoustic data from Antarctic minke whales, Risch dove at the chance to investigate.

"It was very exciting. When going through the recordings, my heart jumped a little bit when I found a sound that was a good candidate for the signal," Risch, whose findings were published April 22 in the journal Biology Letters, said in the national Geographic piece..

Weighing between six and nine tons and measuring approximately 25 to 35 feet in length, Antarctic minke whales, use baleen to filter out krill and other small marine animals from the region's frigid waters. They typically spend their winters in warmer ocean environments nearer the tropics and then return to Antarctic waters to breed during the summer.

Like many marine mammals, the whales were assumed to communicate by calling each other underwater. In fact, the calls of the Antarctic minke had never been formally recorded and analyzed.

So last year, Risch and her colleagues took a boat into Wilhelmina Bay, off the western Antarctic Peninsula, and used a simple recording device attached to a pole to capture the calls of two wild minke whales.

And after all the years of scratched heads and creative speculation Risch heard from large volume of sounds made by the two creatures, several distinct calls they were identifies, with little doubt, as belonging to the elusive "bio-duck."

Of course, now Risch has taken on the mission of discovering why the whales make the seeming quacking noises, as well as why they're not heard throughout the year.